In recent years, an optical fiber transmission has used coherent reception in which a local oscillation laser is provided in a receiver to mix a received optical signal and an electric field, and been able to transmit and receive two bits or four bits per symbol using multilevel phase shift keying such as Quadrature Phase Shift Keying (QPSK) and 16 Quadrature Amplitude Modulation (16QAM).
A laser of a transmitter and a local oscillation laser of a receiver randomly vary in their relative phase. Accordingly, a disclosure is made of an art to extract a phase-modulated signal by converting a coherently received signal to a digital signal through an analog-to-digital converter (hereinafter, referred to as AD converter), performing digital signal processing using a digital signal processing device, to remove random variations in the relative phase between a laser of a transmitter and a laser of a receiver.
Meanwhile, a digital signal processing device allows an easy execution of digital signal processing, which enables coping with various types of transmission deterioration by sending auxiliary control information for a successful transmission of a primary signal representing primary information to be sent.
For example, compensation for a linear waveform distortion such as wavelength dispersion and a nonlinear waveform distortion in an optical fiber transmission line is able to be performed in the transmitter as well as the receiver. Specifically, for the wavelength dispersion among the waveform distortion, the transmission deterioration of the primary signal can be improved by optimally adjusting the compensation amount in the transmitter and receiver respectively than compensation in either one of the transmitter or receiver. In this case, the receiver detects an amount to be compensated and sends the detection result to the transmitter as auxiliary information, which information can be utilized by the transmitter for calculating the compensation amount.
Additionally, for another example, notifying the receiver of a modulation scheme as the control information from the transmitter enables the digital signal processing device to be used to switch a modulation scheme according to a situation of the transmission deterioration and to receive the primary signal in the modulation scheme switched.
Non-Patent Literature 1 discloses a technique of superimposing frequency-modulated auxiliary control information into a primary signal to transmit the auxiliary control information for the primary signal.